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Fabrication, Characterization and Thermo-

physical Property Evaluation of SiC Nanofluids

for Heat Transfer Applications

Nader Nikkam1, Mohsin Saleemi1, Ehsan B. Haghighi2, Morteza Ghanbarpour2, RahmatollahKhodabandeh2, Mamoun Muhammed1, Bjorn Palm2, Muhammet S. Toprak1,∗

Received 05 December 2013; accepted 26 January 2014; published online 20 March 2014)

Abstract: Nanofluids (NFs) are nanotechnology-based colloidal suspensions fabricated by suspending

nanoparticles (NPs) in a base liquid. These fluids have shown potential to improve the heat transfer prop-

erties of conventional heat transfer fluids. In this study we report in detail on fabrication, characterization

and thermo-physical property evaluation of SiC NFs, prepared using SiC NPs with different crystal structures,

for heat transfer applications. For this purpose, a series of SiC NFs containing SiC NPs with different crys-

tal structure (α-SiC and β-SiC) were fabricated in a water (W)/ethylene glycol (EG) mixture (50/50 wt%

ratio). Physicochemical properties of NPs/NFs were characterized by using various techniques, such as pow-

der X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM),

Fouriertransform infrared spectroscopy (FTIR), dynamic light scattering (DLS) and Zeta potential analysis.

Thermo-physical properties including thermal conductivity (TC) and viscosity for NFs containing SiC particles

(α- and β- phase) weremeasured. The results show among all suspensions NFs fabricated with α-SiC particles

have more favorable thermo-physical properties compared to the NFs fabricated with β-SiC.The observed dif-

ference is attributed to combination of several factors, including crystal structure (β- vs. α-), sample purity,

and residual chemicals exhibited on SiCNFs. A TC enhancement of ∼20% while 14% increased viscosity were

obtained for NFs containing 9 wt% of particular type of α-SiC NPs indicating promising capability of this kind

of NFs for further heat transfer characteristics investigation.

Keywords: SiC nanoparticles; Nanofluids; Thermal conductivity; Viscosity; Thermo-physical property

Citation: Nader Nikkam, Mohsin Saleemi, Ehsan B. Haghighi, Morteza Ghanbarpour, Rahmatollah Khod-

abandeh, Mamoun Muhammed, Bjorn Palm and Muhammet S. Toprak, “Fabrication, Characterization and

Thermo-physical Property Evaluation of SiCNanofluids for Heat Transfer Applications”, Nano-Micro Lett.

6(2), 178-189 (2014). http://dx.doi.org/10.5101/nml.v6i2.p178-189

Introduction

Heat transfer fluids are involved in many industrialprocesses to remove excess heat. Conventional heattransfer fluids such as water (W), ethylene glycol (EG)and their mixture (W/EG) have shown poor thermalconductivity (TC) characteristics. On the other hand,developing effective cooling methods for high-tech ap-

plications such as transportation, microelectronics, cut-ting fluid, energy supply, manufacturing and biomedicalapplications have been prioritized to accelerate furtherdevelopment of these kinds of technologies [1-6]. Hence,conventional working fluids for cooling are being re-placed with newer fluids with enhanced TC than thepure and conventional liquids. Pioneer investigationsof Choi [7], Lee and Choi [8] and Masuda et al. [9]

1Department of Materials and Nano Physics, KTH Royal Institute of Technology, SE-16440 Kista-Stockholm, Sweden2Department of Energy Technology, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden*Corresponding author. E-mail: [email protected]

Nano-Micro Lett. 6(2), 178-189 (2014)/ http://dx.doi.org/10.5101/nml.v6i2.p178-189


reported that the effective TC of suspensions contain-ing suspended nanoparticles (NPs) can be higher thanthose of normally used industrial heat transfer liquids.Such kind of new fluids, called as nanofluids (NFs), isa new class of dispersions containing nano-sized solidparticles (NPs) dispersed in conventional heat trans-fer fluids as base liquid [7]; and are considered to benew enhanced heat transfer fluids, since they might of-fer appealing possibilities due to their improved heattransfer performance [10-14]. Consequently, there is ademand for effective and novel heat transfer fluids tosolve these challenges. During last decade, there hasbeen a rapid development in NFs preparation methods.There are two major techniques, which are usually uti-lized for making NFs: one-step method and two-stepmethod. In the one-step method [10,15], NPs are di-rectly prepared in the base liquid, thus forming the NFsin a combined process. Usually, physical vapor depo-sition (PVD) technique [10], solution chemical method[15] or microwave-assisted route [16] is used for one-step

preparation of NFs. There are some advantages in thispreparation method, including minimum agglomerationand better stability. In the two-step fabrication method[17-18], NPs are first synthesized/acquired, followed bydispersing them into the base liquid (such as W or mix-ture of W and EG) by techniques including high shear[19-20] and ultrasound [21]. Currently, availability ofNPs from commercial sources and different suppliershas attractedresearchers to this method, since they canhave several choices for a certain nanoparticle, whichallow effective and comparative research.

To date, several kinds of solid particles/NPs and baseliquids have been utilized in order to fabricate NFs forefficient heat transfer applications: NPs of metals suchas Ag [22], Cu [23] and Au [24], oxides such as Fe3O4,mesoporous SiO2, CuO and Al2O3 [25-28] and carbonbased materials such as CNTs [29] are some examples inthis regard. Among various materials used for prepar-ing NFs, SiC is one of the promising materials to fab-ricate efficient NFs for cooling applications. It has one

Table 1 Summary of available literature on thermo-physical properties of SiC NFs

MaterialBase

Liquid

NP Loading

(wt%)pH Size (nm) T (◦C)

% TC

Enhancement

% Increase

in ViscosityRef.

α-SiC W/EG 13 9.4

16�

29�

66�

90�

22.5

1.5

15

15.5

17

52

35

22

17

[35]

α-SiC Water 13 9.4

16�

29�

66�

90�

22.5

7

9.7

11.7

12.5

60

38

18

17.5

[34]

SiC Water

3

6.3

9.5

11

22.5

22.5

22.5

3.7

4.2

7.2

102

–

–

[32]

α-SiC

24

3

6

12

9-10 170∗70

22.5

28

4

9

22

260

38

185

[33]

α-SiC Water 13.2 10

26�

(spherical)

600�

(Cylinder)

4

4

9

16Not Reported [31]

α-SiC EG 13.2 –

26�

(spherical)

600�

(Cylinder)

4

4

10

16Not Reported [31]

*Data obtained by DLS Measurements�Data estimated from BET measurements

179


of the highest TC values (360-490 W/m·K; dependingon crystal structure of SiC) [30] and is commerciallyavailable. Table 1 shows the summary of the litera-tureson SiC NFs for heat transfer applications. For thefirst time, Xie et al. [31] investigated TC of SiC NFs at4◦C and reported some TC enhancement methods, yetthey did not report on the viscosity of NFs. Won Leeet al. [32] developed water based NFs with β-SiC NPsand investigated TC and viscosity of suspensions at 9.5wt% particle loading. Singh et al. [33] worked on wa-ter based commercial suspension containing α-SiC NPsand reported about TC and mechanical effects of SiCNPs in water media. Timofeeva et al. [34] studied theeffect of different particle size and interface on thermo-physical properties and heat transfer characteristics ofwater based NFs with α-SiC NPs. In another study,Timofeeva et al. [35] reported on a series of NFs withα-SiC NPs indifferent base liquids (water and W/EGmixture) and NPs sizes. They have investigated the ef-fect of temperature and base liquid on the heat transfercharacteristics of water and W/EG based α-SiC NFs.

Although few studies have been performed on theuse of SiC NF for heat transfer applications, there isno comparative study onthe effect of SiC crystal struc-ture on the thermo-physical properties of NFs contain-ing α-SiC and β-SiC NPs. Therefore, the objective ofthis work is to fabricate SiC NFs with different SiC-crystalline phases in order to investigate comparativelyphysicochemical and thermo-physical properties includ-ing TC, and viscosity of SiC NFs in W/EG as the baseliquid.

Experimental

Materials and methods

Silicon Carbide (SiC) particles with different crys-tal structures of alpha type (α-SiC) and beta type (β-

SiC)were purchased from various suppliers/institutesincluding Superior Graphite (USA), PlasmaChemGmbH (Germany) and ENEA (Italy). Ethylene gly-col (EG) and sodium hydroxide (NaOH) were acquiredfrom Sigma Aldrich and Merck KGaA (Germany). Allthe reagents were used as received without further pu-rification. The materials are abbreviated for ease asshown/listed in Table 2.

Fabrication of α-SiC NF and β-SiC NF via two-step method

A series of NFs were fabricated by dispersing a knownweight of α-SiC and β-SiC NPs in W/EG (50:50) baseliquid, via two-step method. The suspension was son-icated for 15 min and the pH value was adjusted to∼9.5 using NaOH in order to obtain stable NFs con-taining 9 wt% α-SiC and β-SiC NPs. All NFs (Table3) were stable without any visual precipitation withina week. In order to study the real effect of α-SiC and β-SiC NPs on the thermo-physical properties of NFs, NPswere used as received and the use of surfactant/surfacemodifiers was avoided. Fabricated NFs were evaluatedfor TC and viscosity properties. Formulating NFs forobtaining optimum thermo-physical properties such asoptimal TC enhancement, minimum impact on viscos-ity and longer stability is always desired. In this case,several factors such as selection of optimum particleloading play essential role. In this study, the optimumparticle loading (9 wt%) was selected after an exten-sive test campaign of NFs with various NP concentra-tion, where optimizing basis was maximizing the TCenhancement of NFs with minimal impact of NPs ad-dition on viscosity. Moreover, by doing the literaturesurvey about the relevant research (Table 1) concentra-tion of 9 wt% was selected in order to achieve NFs withoptimal properties. Therefore, all NFs were made withNPs loading of 9 wt%. As Table 1 shows, at higher

Table 2 SiC NPs tested for TC and viscosity

Material Supplier/Institute NP ID Surface Area∗ (m2/g)

SiC-Alpha type Superior Graphite (USA) (α1-SiC) 18

SiC-Alpha type PlasmaChem GmbH (Germany) (α2-SiC) 18

SiC-Beta type PlasmaChem GmbH (Germany) (β1-SiC) 80

SiC-Beta type ENEA (Italy) (β2-SiC) 90

*Data provided by the supplier.

Table 3 SiC NFs tested for TC and viscosity properties

Sample NP ID Crystal Structure Base Liquid (50:50) by wt% NPs loading (wt %) pH

α1-SiC NF-W/EG (α1-SiC) Hexagonal W/EG 9 ∼9.5

α2-SiC NF-W/EG (α2-SiC) Hexagonal W/EG 9 ∼9.5

β1-SiC NF-W/EG (β1-SiC) Cubic W/EG 9 ∼9.5

β2-SiC NF-W/EG (β2-SiC) Cubic W/EG 9 ∼9.5

180


concentrations, although NFs with higher TC wasachieved, very high viscosity values were observed forNFs, which is unfavorable for their use in practical heattransfer applications.

Characterization techniques

Microstructure and morphology of α-SiC and β-SiCparticles were evaluated by using scanning electron mi-croscopy (SEM; FEG-HR Zeiss-Ultra 55). Transmis-sion electron microscopy (TEM) analysis of the par-ticles was performed using JEOL 2100 at 200 kV ac-celeration voltage. Nicolet Avatar IR 360 spectropho-tometer, in the range of 500-4000 cm−1 was used forFourier transform infrared spectroscopy (FTIR) analy-sis of solid particles and solid/liquid samples. PowderX-ray diffraction (XRD) was performed on a PhilipsX’pert pro super diffractometer with Cu Kα source(λ=1.5418 A). Zeta potential analysis of α-SiC and β-SiC particles was performed for evaluating NFs stabil-ity region. Average solvodynamic particle size distri-bution of β-SiC particles was evaluated by Beckmann-Coulter DelsaNano C system. TC of NFs was mea-sured by using TPS 2500 instrument (HotDisk model2500), which works based on the Transient Plane Source(TPS) method. The validity of the TPS instrumentwas checked by comparing with a standard source forthermodynamic properties of water (IAPWS reference)[36]. Compared to the reference the accuracy of mea-surement for distilled water was within 2% [36]. Fi-nally, viscosity of NFs was evaluated using a DV-II+Pro-Brookfield viscometer.

Results and discussion

XRD analysis

There are about 250 crystalline forms for Silicon

carbide material [37]. Figure 1(a) displays β-SiC, whichis formed at temperatures below 1700◦C, has a cu-bic crystal structure [38], while as Fig. 1(b) shows theschematics of α-SiC, which is formed at temperatureshigher than 1700◦C, has a hexagonal crystal structure.In order to identify the crystal structure of SiC NPs,XRD analysis was performed. Figure 1(c) shows theXRD pattern of β1-SiC and β2-SiC NPs. The diffrac-tion peaks at 35.8◦, 41.5◦, 60.0◦, 72.0◦ and 75.7◦ can beindexed as the (111), (200), (220), (311) and (222) re-flections of β-SiC phase, respectively (JCPDS#01-074-2307). The diffraction peak at 34.5◦ is attributed to asmall amount of α-SiC phase. β2-SiC sample exhibitsexcess Si phase and all the extra peaks were indexedwith Si reference pattern (JCPDS#00-027-1402). Fig-ure 1(d) presents the XRD patterns of α1-SiC and α2-SiC particles. Both types of α-SiC show hexagonal crys-tal structure and all observed peaks were indexed withthe JCPDS # 01-073-1663 for hexagonal SiC phase.

Morphology analysis

SEM micrographs of α-SiC and β-SiC NPs are pre-sented in Fig. 2(a)-(d). Figure 2(a) and 2(c) representthe spherical morphology of β-SiCwith estimated pri-mary particle size of (60±10) nm and (30±10) nm forβ1-SiC and β2-SiC, respectivelywhile Fig. 2(b) and 2(d)present the SEM micrographs of α1-SiC and α2-SiC, re-spectively. Since there is a very wide size dispersion forboth α1 and α2-SiC NPs, it is rather difficult to per-form size estimation from the micrographs; however, arough estimate (by counting > 200 particles) showedthe primary particle size of (115±35) nm and (85±20)nm for α1-SiC and α2-SiC, respectively. SiC NPs withα-type structure shows larger particle size than β-typeSiC NPs, which may influence the thermophysical prop-erties of the resultant NFs.

20 30 40 50 60 70 80

(c)

(311

)

(400

)

(220

)

(111

)

(222

)

(311

)

(220

)

(200

)

JCPDS# 01-074-2307

Si(JCPDS# 00-027-1402)

(B)

(A)

(A) β1-SiC

α-SiC(B) β2-SiC

(A) α1-SiC

(B) α2-SiC

2θ (degree)20 30 40 50 60 70 80

2θ (degree)

Inte

nsi

ty (

a.u.)

(111

)

JCPDS # 01-073-1663

(205

)(2

04)

(203

)(116

)

(109

)(108

)

(107

)

(105

)

(104

)(103

)

(006

)

(A)

Inte

nsi

ty (

a.u.)

(B)

(101

)

(d)

(a) (b)

Fig. 1 The unit cell (crystal structure) and XRD pattern of (a, c) β-SiC and (b, d) α-SiC particles, respectively.

181


100 nm

100 nm 100 nm

100 nm

(a)

(c)

(b)

(d)

Fig. 2 SEM micrographs of SiC NPs for sample IDs: (a) β1; (b) α1; (c) β2; (d) α2 (all scale bars are 100 nm.)

50 nm 100 nm

100 nm 100 nm

(a) (b)

(c) (d)

Fig. 3 TEM micrograph of SiC particles for samples: (a) β1, (b) α1, (c) β2 (inset: SAED pattern), (d) α2 (inset: SAEDpattern) (all scale bars are 100 nm.).

Figure 3(a)-(d) show the TEM micrographs of β-SiCand α-SiC NPs, respectively. Near spherical morphol-ogy for β-SiC NPs was observed (Fig. 3(a) and 3(c))while hexagonal morphology for α-SiC particles wereclearly visible (Fig. 3(b) and 3(d)). The morphology ofα-SiC particles may be dominated by the anisotropiccrystal structure, allowing the crystal to grow in cer-tain directions more than the other directions (a.k.a.Crystal habit). Compared to the SEM micrographspresented in Fig. 2(a) and 2(c), β-SiC NPsin TEM mi-

crographs highly agglomerated but much smaller par-ticles were observed. Selected area electron diffraction(SAED) pattern, shown in inset images in Fig. 3(c) and3(d), were indexed for cubic structure β-SiC (ICDDNo: 01-074-2307) and hexagonal crystal structure α-SiC (ICDD No: 01-073-1663), respectively. These elec-tron diffraction results reveal purity and good crys-tallinity of β-SiC NPs and α-SiC particles. Moreover,there is good match between SAED pattern and XRDanalysis.

182


Dynamic light scattering (DLS) analysis

DLS analysis was performed to estimate the dis-persed size of SiC NPs in liquid media, in order to un-derstand the influence of effective size of dispersed NPsin the liquid media. DLS analysis results are shown inFig. 4(a) and 4(b) for both α and β-SiC in pure waterand W/EG media. Figure 4(a) displays DLS results forα1-SiC and α2-SiC in pure water and W/EG mixture asbase liquids. We included pure water in our DLS analy-sis, since it is also one of the most commonly used baseliquids. A wide range of particle size distribution (150-4500 nm) with an average peak value of ∼1290 nm wasobtained for α1-SiC particles in water media. For α2-SiC in water media, a much wider range of size between30-10000 nm, with an average peak value of ∼1800 nmhas been obtained, indicating strong aggregation of par-ticles in water base liquid. When it comes to W/EGmedia, both in case of α1- and α2-SiC, smaller aver-age sizes with narrower size distribution was observedwhen compared to the water media, indicating W/EGcan affect significantly the dispersion property of SiCparticles. EG may act as dispersant and covalentlybond onto the particles surface, which allow stabiliza-tion of smaller aggregates/agglomerates as compared topure water media. In comparison, α2-SiC NPs showedlarger dispersed particle size than that of α1-SiC. Theaverage dispersed size of β-SiC NPs in both liquids arepresented in Fig. 4(b). When β-SiC NPs were dispersedin pure water it is seen that, the size of β1-SiC NPs isin the range of ∼100-800 nm with an average diame-ter of 340 nm while the size estimation for β2-SiC inwater media showed the range of 130-450 nm with anaverage hydrodynamic diameter of ∼260 nm. A nar-row NPs size distribution with smaller average hydro-dynamic size was obtained for β2-SiC NPs. In W/EGmedia, the size of β1-SiC NPs is in the range of 100-450nm with a peak average of 245 nm while those rangeand average number are 100-200 nm and 145 nm forβ2-SiC NPs. A narrower NPs size distribution with asmaller average dispersed size is estimated for β2-SiCNPs. In the same way, as observed for α-SiC NPs,the presence of EG reduces the aggregate/agglomeratesize in the suspensions. Looking at the all results pre-sented in Fig. 4(a) and 4(b), β-SiC NPs have smallerdispersed size than α-SiC NPs, both the NPs exhibitsmaller average solvodynamic size in W/EG base liquidcompared to the pure water media, indicating a betterdispersability in W/EG media. A comparison betweenSEM, TEM and DLS results for β-SiC NPs display thatfor all the α and β-SiC NPs the primary particle sizeobtained from SEM and TEM micrographs is less thanthe predicted sizes from DLS method. This differencemay be due to the aggregation/agglomeration of α-SiCand β-SiC NPs in the base liquid media, in addition tothe adsorbed liquid layer on particles’ surface.

10 100 1000 100000

1

2

3

4

5

6

(a)

(b)

Inte

nsi

ty

Diameter (nm)

0 200 400 600 800 10000

5

10

15

20

25

Inte

nsi

ty

Diameter (nm)

α1-SiC-W/EGα2-SiC-W/EGα1-SiC-Waterα2-SiC-Water

β1-SiC-W/EGβ2-SiC-W/EGβ1-SiC-Waterβ2-SiC-Water

Fig. 4 Particle size distribution of (a) α-SiC (b) β-SiC NPsin water and W/EG base liquids measured by DLS.

FTIR analysis

In order to study the surface characteristics of parti-cles, FTIR analysis was carried out. FTIR spectra forall “as-received” SiC particles are presented in Fig. 5.The absorption band between 860 cm−1 and 760 cm−1

is attributed to the Si—C bond. For all the SiC parti-cles, the absorption between 1100 cm−1 and 1000 cm−1

are assigned to Si—O—Si or Si—O—C vibrations, re-spectively. For α1-SiC particles theband at 1200 cm−1

is attributed to the Si—C. α2-SiC particles show ab-sorption bands at 1180 cm−1 and 1380 cm−1, which canbe attributed to the Si—C and amorphous carbon, re-spectively. Moreover, in both α1-SiC and α2-SiC parti-cles, there are two bands between 1525 and 1620 cm−1,attributed to the C==O groups, which may be due tothe un-reacted precursor or residual chemicals due tothe method used for their fabrication. By looking at β1-SiC, in addition to Si—C bond Si—O—Si at 780 and1050 cm−1, respectively. There are two more bandsat 1200 and 1315 cm−1. The first band is assignedto Si—C and the second one to amorphous carbon, re-spectively. As Fig. 5 shows, compared to other samples,β1-SiC and α2-SiC (both are from the same supplier)present shoulder around 800 cm−1, which may be at-tributed to the presence of O at the interface due tothe synthesis procedure of high-temperature reductionof SiO2, based on information provided by the supplier

183


3500 3000 2500 2000 1500 1000

20

40

60

80

100

Tra

nsm

itta

nce

(%

)

Wavenumber (cm−1)

Shoulder

α1-SiCα2-SiCβ1-SiCβ1-SiC

Si−−O−−SiSi−−C−−C==O

Fig. 5 FTIR spectra of as-received β- and α-type SiC par-ticles.

[39].

Zeta potential analysis

Characterizing NPs dispersions and understandingthe role of various parameters, which may affect col-loidal properties, are important for any NFs. In theliterature, very limited investigations can be found onNFs that report on the effect of pH, particle surfacechemistry, primary particle size and crystal phase onproperties such as Isoelectric Point [40-46]. The pH ofa suspension has important role not only on the rheo-logical property of suspension but also in terms of fab-rication of stable suspension, which is related to theelectrostatic charge on particles’. It is well known thatfor fabricating stable NFs, the pH value of the NF mustbe far from the Isoelectric Point (IEP), where the over-all charge on the particles becomes zero. If the pHgets close to the IEP the particles tend to agglomerateand finally precipitate, since there are not enough re-pulsive forces between them. When the pH is set farfrom the IEP the absolute electrical charge on particlesis increased, which cause repulsive interaction betweenparticles due the collision. In order to identify the op-timum pH values for stable NFs formulation, Zeta po-tential analyses were performed for both the α-SiC andβ-SiC particles in the pH region from 2 to 10. The re-sults are presented in Fig. 6 where the IEP obtainedfor the α-SiC and β-SiC particles show great variations,one very important common point of exhibiting highlynegatively charged particles in the pH region of ∼9.5and 10. Therefore, the pH of NFs was adjusted at 9.5to obtain stable suspensions. Figure 6 indicates thatβ-SiC NPs has higher IEP values compared to the α-SiC. The α1-SiC and α2-SiC particles have very closeIEP values in the pH range of 2-3, while β-SiC NPsdisplay similar IEP values in the pH range of 5-6. Thepossible reasons for this observation may be due to dif-ferent synthesis methods of SiC particles, and impu-rities present, resulting in different surface chemistryof SiC particle with α- and β- type. As FT-IR analy-

sis revealed (Fig. 5), different SiC NPs exhibited differ-ent surface chemistry. By doing a comparison betweenFT-IR test results and Zeta potential analysis (Fig. 6),one can see that having lower IEP for α2-SiC than α1-SiC may be due to higher silica content (more intensiveSi—O—Si bands in FTIR) of α2-SiC, which shifts theIEP to more acidic pH region (IEP of SiO2 is about 2)[47]. The β2-SiC NPs exhibit slightly lower IEP thanβ1-SiC NPs. Although β2-SiC showed higher Si—O—Sibands in FTIR, it exhibited lower IEP due to probablythe high content of Si (assessed from XRD).

2 4 6 8 10

α1-SiCα2-SiCβ1-SiCβ1-SiC

12−50

−40

−30

−20

−10

0

10

20

30

40

Zet

a pot

enti

al (

mV

)

pH

Fig. 6 Zeta potential as a function of pH for β- and α- typeSiC particles.

There are few studies about the NPs’ size effect onsuspension properties, particularly IEP. The size of par-ticles may not only affect the IEP of the suspensionbut also the surface reactivity, adsorption affinity andphotocatalytic activity of NPs [45-46, 48-49]. As NPssize decreases, the percentage of surface atom/moleculeincreases extensively. Particle electronic structure, sur-face defect density and surface sorption sites can be alsochanged [50-51]. Therefore, both surface reactivity andNPs IEP can become dependent on particle size. Thereis no report in literature about the impact of SiC NPscrystal on suspension properties such as IEP. Althoughthere is no systematic study about size effect on SiCsuspension properties yet, there are several reports onTiO2 suspensions and the influence of materials crystalstructure on suspension properties [42,46]. Kosmulski[42] reported on the effect of crystal phase of TiO2 onIEP and showed that the IEP is rather independentto the crystal structure (rutile vs anatase). The sameobservation was also repeated by Suttiponparnit et al.

[46] when their experimental efforts confirmed that theIEP of TiO2 NPs was insensitive for the crystal struc-ture of TiO2.

Suttiponparnit et al. [46] studied the role of primaryNP size on TiO2 dispersions with different crystal struc-tures (anatase and rutile) and showed that when pri-mary particle size increased, the IEP decreased. In ourwork, with a focus on size effect on SiC NPs, it canbe seen that at the same particle loading of SiC with

184


α type when the primary NP size increased the IEPalso increased (primary size has not been calculatedfor α-SiC as the particles are non-spherical, though themagnitude of BET surface area is used for size compar-ison; large BET surface area revealing smaller primaryparticles – c/o Table 1). The same point is valid forβ-SiC, for which an increase of IEP was observed withincreasing primary NP size. A two-three orders of mag-nitude difference between the IEP values of α-SiC andβ-SiC NPs may also be attributed to be sensitive to theSiC NPs crystal structure/phase. Our results on IEPof SiC NFs showed that IEP may be affected by theprimary size of dispersed particles, impurities involved,and different surface chemistry (due to the fabricationmethod of particles) of NPs.

TC and viscosity measurements of NFs

Thermo-physical properties of NFs including TC andviscosity were performed for the all fabricated NFs at20◦C. In order to evaluate the TC of NFs, 9 wt% NFswith α-SiC and β-SiC and W/EG as base liquid wereprepared and TPS method was used to test the TC ofNFs. The TC evaluation results, measured at 20◦C, arelisted in Table 4, showing higher TC values (Knf ) of allSiC NFs than the base liquid W/EG. Moreover, Table4 shows that TC of NF with α type SiC are higher thanthat of β types. The reason is not clear, however, sev-eral factors may play the role for facing this difference.It may be due to the dissimilarcrystal structureof SiCNPs which also have different magnitude of TC (360W/m·K for β-and 490 W/m·K for α- type) [30]. Thispoint indicates the possibility of influence of SiC NPscrystal phase on TC of NFs.The size of NPs may alsohave a role. The TC results clearly displayed that for alltype of SiC NPs the TC values increase with increasingNP size, which is compatible with the literature [52].

Even sample impurityand different surface chemistryof SiC NPs, as observed from XRD and FTIR analy-sis,may also result in different TC for NFs. Table 4 alsoshows that the α1-SiC NF exhibited higher TC valuesthan α2-SiC NF. As a result, NF with α1-SiC showedthe highest TC values, while NF containing β2-SiC re-vealed the minimum TC value which may be due tothe all above mentioned reasons such as having largestand smallest particle size for α1-SiC NPs and β2-SiCNPs, respectively. Table 4 lists the relative TC, defined

as the ratio of TC of SiC NFs (Knf ) over the TC ofbase liquid (Kbl). The maximum TC enhancement of20% was obtained for α1-SiC NF at 20◦C. Timofeevaet al. [35] presented several results for α-SiC NFs withW/EG base liquids (Table 1). In the best case theyreported 17% TC enhancement for the particles withaverage size of 90 nm. A comparison between the TCenhancements of α1- SiC NF- W/EG (present study)at 20◦C and that reported by Timofeeva et al. [35],where they used commercial NFs, shows that NFs forthe present work has higher TC even at ∼4 wt% lowerparticle loading, indicating higher effective thermal per-formance of NF presented in this work.

It is important to clarify that having greater relativeTC value only is not enough for utilizing a NF as effec-tive coolant. In order to choose the efficient NF withoptimum characteristics for heat transfer applications,not only TC but also viscosity must be evaluated. Theinternal resistance of a fluid to flow is described by vis-cosity [53], which plays an important role in all thermalapplications involving flowing fluids [54]. This propertyis expected to be higher when compared to their baseliquids, but this increase makes a negative impact onthe pumping power and heat transfer coefficient. Theseparameters are very essential in practical heat transferapplications. For instance, in laminar flow, the pressuredrop is directly proportional to the viscosity. Moreover,Prandtl and Reynolds numbers are influenced by viscos-ity of fluids and heat transfer coefficient is a functionof these numbers. Therefore, viscosity is as importantas TC in engineering systems involving fluid flow [55].This is an important criterion for the use of this type ofNFs in convective heat transfer applications. The vis-cosity tests were carried out at 20◦C for all NFs withW/EG as the base liquid,where all the samples exhib-ited Newtonian behavior. The results are listed in Ta-ble 4, which show that all NF with α- type SiCparticleshave lower viscosity values than NF containing β- typeSiCparticles. Three possible reasons can be enlisted forthe observed difference as the effect of the crystal struc-ture or difference in primary particle size(BET surfacearea) for α-SiC NF and β-SiC NPs and even to the levelof impurities as observed from XRD and FTIR analy-sis. The smaller surface area of α-SiC particles (Table2) results in a smaller contact (solid-liquid interface)area between α-SiC particles and W/EG base liquidand therefore exhibit smaller viscosity value than

Table 4 TC and viscosity evaluation results of SiC NFs tested at 20◦C

Sample NPs loading (wt%) TC (W/mk) Knf /Kbl Viscosity (cP) μnf /μbl

Base Liquid (W/EG) – 0.392 – 3.897 –

α1-SiC NF W/EG 9 0.4704 1.20 4.4425 1.14

α2-SiC NF W/EG 9 0.4626 1.18 4.3646 1.12

β1-SiC NF W/EG 9 0.4469 1.14 5.299 1.36

β2-SiC NF W/EG 9 0.4195 1.07 6.2352 1.60

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β-SiC at the same particle loading. The increased vis-cosity values for α1-SiC and α2-SiC particles are veryclose as the surface area for both particles are almostthe same. Secondly, for the same reason α2-SiC NFexhibited the lowest and β2-SiC NFs the highest vis-cosity values at 20◦C. The relative viscosity, which isdefined viscosity of NF (μnf ) to the viscosity of base liq-uid (μbl), are listed in Table 4. The minimum increaseof ∼12% in viscosity was achieved for SiC NF with 9wt% α2-SiC particle loading, which has small surfacearea (larger average particle size), while the maximumincrease of ∼60% in viscosity was obtained for SiC NFwith β2-SiC at the same particle loading. NFs contain-ing β2-SiC exhibit higher increase in viscosity comparedto the β1-SiC NF at the same NPs loading may be dueto its smaller size, and impurity content, which pro-vides a larger surface area per unit volume. A directcomparison between increase of viscosity in this workand the reported values in the literature (Table 1) isnot possible because of having different factors suchas particle loadings, particle size and dissimilar tem-perature. Selecting NFs with similar particle loadings,Won Lee et al. [32] reported 7.2% TC enhancementwhile increase in viscosity was 102% for a water basedβ-SiC NF with ∼9.5 wt% particle loading indicatingthat although it exhibit nearly the same relative TCcompared to the worst case in this study (β2-SiC NFwith 9 wt% NPs loading), shows 42% more increase inviscosity. Our findings show favorable thermo-physicalproperties (higher TC enhancement with lower increasein viscosity) compared to all series of water based α-SiCNFs reported by Timofeeva et al. [34] (Table 3). Theyhave utilized commercial water based NFs, preparedby dispersing α-SiC NPs, and reported 7-12.5% TC en-hancement with 17.5-60% increase in viscosity of theNFs containing 13 wt% of SiC NPs with different sizes.Timofeeva et al. [35] also reported on α-SiC NFs withW/EG showing 17% increase in viscosity at 13 wt%SiC NPs loading (Table 1). The observed differencesbetween this work and Timofeeva et al [34,35] may bedue to the different surface chemistry of α-SiC particles(IEP= 4 [34]), type of the base liquid, or additives, be-sides different NPs loading. The increased viscosity willresult in use of a greater pumping power, which mightcounterweigh the benefits of higher TC enhancementvalues. The tradeoff between thermo-physical proper-ties including increases in viscosity values and relativeTC is very essential in order to utilize the NFs for heattransfer applications.

Figure 7 summarizes the comparison between TCenhancement and viscosity increase, for all fabricatedNFs, where both α-SiC NFs show that the TC increaseis higher than the viscosity increase at 20◦C, while re-verse behavior is observed for β-SiC NFs. This mayimply that α-SiC NFs formulations can be proper can-didates as efficient NFs for convective heat transfer ap-

plications. Moreover, a comparison between α1-SiC NFand α2-SiC NF shows that although α1-SiC NF has 2%higher viscosity increase, it exhibits 2% greater TC en-hancement value indicating a promising capability ofthis NF as efficient coolant in heat transfer applica-tions. Among all the fabricated NFs in this work, NFsfabricated by α1-SiC particle is the optimum choice,which is therefore selected for further thermal charac-teristics investigations, and results are to be reportedelsewhere.

0

10

20

30

40

50

60

70

Knf/

Kbl &

μnf/μ

bl

Knf/Kbl

μnf/μbl

α1-SiC NF α2-SiC NF β1-SiC NF β2-SiC NF

Fig. 7 Comparison of the TC enhancement with increasein viscosity (at 20◦C) for NFs, with W/EG as base liquid,containing 9 wt% α- and β- type SiC particles.

Conclusions

We presented on the fabrication and evaluation ofhighly stable SiCNFs for heat transfer applications.The NFs were prepared by dispersing SiCparticleswith different crystal structure in W/EG base liq-uid, using colloidal stabilization strategies. A detailedphysico-chemical evaluation showed different charac-teristics of SiC NPs, including crystal structure, pri-mary/dispersed particle size, surface functionality, sur-face charge, and purity levels of SiCNPs used. Inter-pretation of the results from different analytical tech-niques showed that IEP of the SiC NPs, and the viscos-ity of NFs, may be affected by the primary size, surfacechemistry of NPs (due to the different NPs fabricationroutes), as they may also be dependent on SiC crystalstructure (α- vs. β- type). Thermo-physical properties,TC and viscosity, of SiC NFs were performed at 9 wt%NPs loading at 20◦C. TC enhancement of the NF, overthe base liquid, due to the presence of SiC particles areobserved for NFs containing both α- and β-type SiC-NPs. W/EG based NFs with α-SiC exhibited higherTC than that with β-SiC, which may be attributed tothe effect of the crystal structure (as α-type has higherTC value), or the phase purity of the as-received mate-rials. Among all fabricated W/EG based NFs contain-ing SiCwith different crystal structure (α- vs. β- type),α1-SiC NF displayed the highest TC enhancement of20%,while only 14% increase in viscosity,revealing its

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promising characteristics for heat transfer applications.

Acknowledgments

The financial support from the EU (Project Refer-ence: 228882) and Swedish Research Council (VR) forthe project NanoHex (Enhanced Nano-fluid Heat Ex-change) is highly appreciated.

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